1. Field of the Invention
This invention resides in the field of thin-walled rubber articles, and particularly those that are made from latex and vulcanized to produce both carbon-sulfur and carbon-carbon crosslinking bonds. The goal of this invention is to provide thin-walled rubber articles with favorable tensile characteristics including high tensile strength, high ultimate elongation, and low tensile modulus, and to do so without creating allergic reactions or health concerns that are attributable to some of the chemicals that are commonly used in the manufacture of sulfur-vulcanized rubber.
2. Description of the Prior Art
Natural and synthetic rubber have been used extensively as materials of construction for thin-walled medical devices and personal items. Examples of articles made from these materials are surgical and examination gloves, finger cots, catheter balloons and cuffs, uterine thermal ablation balloons, condoms, contraceptive diaphragms, in-dwelling urinary drainage catheters, male external urinary drainage catheters, breather bags, surgical tubing, baby pacifiers, baby bottle nipples, and drug infusion bladders. Because of the mechanical stresses imposed on these devices during use, the walls of these devices must have a high tensile strength combined with a low 500% tensile modulus. The rubber is vulcanized in any of various ways to achieve structural integrity, but high tensile strength and low tensile modulus are most readily achieved when vulcanization is achieved by the use of sulfur, i.e., by crosslinking of the polymer chains with carbon-sulfur bonds.
The highest durability and flexibility are achieved by a rubber film that is seamless and of uniform thickness. Thin-walled rubber devices formed from latex, particularly by dip-molding, are particularly favorable for these reasons. Latex can be processed without breaking down the molecular weight of the rubber, whereas dry-rubber methods, which utilize high shear to comminute the rubber and combine it with other compounding ingredients for processing, tend to degrade the molecular weight.
Vulcanization with sulfur has traditionally been performed in the presence of sulfur vulcanization accelerators. The first compound found to be capable of accelerating the reaction between sulfur and natural rubber was aniline (first used in 1906), and various other compounds bearing similarities to aniline were subsequently developed that were less toxic and produced greater acceleration activity. Included among these are:                mercaptobenzothiazoles, such as 2-mercaptobenzothiazole, bis(2,2′-mercaptobenzothiazolyl)disulfide, and zinc 2-mercaptobenzothiazole,        sulfenamides, such as N-tert-butyl-2-benzothiazolesulfenamide, N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazolesulfenamide, and 4-morpholino-2-benzothiazolesulfenamide,        dithiocarbamates, such as bismuth dimethyldithiocarbamate, cadmium diethyldithiocarbamate, copper dimethyldithiocarbamate, zinc dimethyldithiocarbamate, and other metal dialkyldithiocarbamates, and piperidinium pentamethylenedithiocarbamate        thiurams, such as dipentamethylene thiuram disulfide, dipentamethylene thiuram hexasulfide, tetramethylthiuram disulfide, tetrabenzylthiuram disulfide, and tetra-n-butylthiuram disulfide,        guanidines, such as diphenylguanidine and di-ortho-tolylguanidine,        thioureas, such as diphenylthiourea, ethylenethiourea, and trimethylthiourea,        xanthates, such as dibutyl xanthogen disulfide and zinc di-iso-propylxanthate, and        dithiophosphates, such as copper O,O-di-iso-propylphosphorodithioate and zinc O,O-di-n-butylphosphorodithioate        
The most widely used accelerators in the above list are those that contain secondary amine groups (RR′N—, as opposed to primary amine groups RNH—), such as dialkyl amino groups, cycloalkylamino groups, and morpholinyl groups. Secondary amine groups are found, for example, among the sulfenamides, the dithiocarbamates and the thiurams. An unfortunate consequence of the inclusion of these accelerators is their tendency to produce an adverse reaction in individuals with whom the articles come into contact. The reaction is commonly referred to as a Type IV allergy, which is mediated by T cells, generally occurs within six to 48 hours of contact with the rubber article, and is localized to the area of the skin where contact is made. Secondary amine-containing accelerators are also referred to as nitrosatable amines since they are susceptible to reaction with atmospheric nitrogen oxides during mixing, milling, extrusion, molding, calendaring, curing, and even warehousing and storage, to produce nitrosamines, which have been designated as potential human carcinogens. Some of these nitrosamines are N-nitroso-di-n-butylamine, N-nitrosodiethanolamine, N-nitrosodiethylamine, N-nitrosodimethylamine, N-nitrosodiisopropylamine, N-nitrosodi-n-propylamine, N-nitrosomorpholine, N-nitrosopiperidine, and N-nitrosopyrrolidine.
Natural rubber itself is responsible for adverse reactions in certain individuals, and these as well are addressed by certain embodiments of this invention. One type of adverse reaction to natural rubber is an indirect reaction that arises as a result of irritant dermatitis. Although not an allergic reaction, irritant dermatitis can cause breaks in the skin which can provide the components of the rubber, including proteins, increased access to the body's immune system and ultimately an allergic reaction. Another type of adverse reaction to natural rubber is a systemic allergic reaction known as a Type I allergy, which is caused by IgE antibodies to the proteins in natural rubber. This is an “immediate” reaction, occurring within thirty minutes of exposure, and its symptoms include hives, rhinitis, conjunctivitis, asthma, and in rare cases anaphylaxis and hypotension.
These adverse reactions to natural rubber can be prevented by using a suitable synthetic rubber. The use of deproteinized natural rubber has been proposed, but it has not been shown that deproteinized natural rubber eliminates the problems entirely. Various synthetic elastomers have been used as well. Nitrile rubber and polychloroprene, for example, have been used in the manufacture of surgical gloves, medical examination gloves, and dental gloves. These materials do not however offer the high resiliency and low tensile set values of natural rubber. Silicone rubber has been used for catheter balloons, but its tensile strength is lower than that of natural rubber and must be compensated for by an increased wall thickness. Polyurethanes have also been used, particularly in dip-molded catheter balloons. Polyurethanes have very high tensile strength, but they lack the resiliency and low tensile set values of natural rubber and are therefore unsuitable for devices that are required to undergo large degrees of expansion during use and then be able to return to their original configuration. Gloves have also been prepared from styrene-ethylene-butylene-styrene tri-block copolymer, but this material has very high tensile set values, a characteristic that causes the glove to exhibit undesirable “bagging,” i.e., to remain stretched after use.
The closest substitutes for natural rubber in terms of overall performance are synthetic cis-1,4-polyisoprene and rubber compositions which are comprised of substantial amounts of synthetic cis-1,4 polyisoprene. There are considerable differences however between synthetic cis-1,4-polyisoprene and natural rubber in terms of molecular structure. The polyisoprene in natural rubber has a molecular weight of from about 1,000,000 amu to about 2,500,000 amu, while the molecular weight of synthetic cis-1,4-polyisoprene ranges from about 250,000 amu to about 350,000 amu (both expressed as number averages). Lower molecular weight polymers generally have lesser tensile properties, including lower tensile strength. Synthetic cis-1,4-polyisoprene also has a lower degree of branching, lower symmetry, and lower intermolecular forces. All of these characteristics contribute to and affect the tensile properties of the polymer.
Certain medical devices, such as surgical and other medical gloves, require a relatively low tensile modulus to remain comfortable during use. If the tensile modulus is too high, the user's hands may become fatigued over time as progressively more strength is required to stretch the glove material. This is particularly problematic with gloves that are to be used for a prolonged period of time such as during a long surgical procedure. The importance of a low tensile modulus is recognized in the standardized testing procedure ASTM D3577, which sets standards for the tensile properties. The standards require that the 500% modulus value be 7 MPa or less for synthetic gloves, and 5.5 MPa or less for natural rubber gloves. Low tensile modulus values are also important for condoms to promote ease of donning, and for catheter balloons where ease of inflation is beneficial. A low tensile modulus is also of value in elastomeric drug infusion bladders by making it easier to fill the bladder with a drug solution.
Another tensile property affecting the usefulness of certain medical and personal devices is tear strength, which is important in preventing premature failure of the device. Baby bottle nipples and baby pacifiers also benefit from high tear strength since this prevents the child's teeth from severing the nipple or pacifier during use. It is generally known that rubbers that are crosslinked only through carbon-carbon bonds have inferior tear strength compared to rubbers that contain sulfidic and/or polysulfidic crosslinks.
A still further tensile property that is important to the satisfactory performance of rubber medical devices is ultimate elongation. Increasing the ultimate elongation value is believed to reduce the incidence of breakage in use. This is of benefit for example to condoms and catheter balloons, as well as to surgical gloves which are easier to don if they have a high ultimate elongation value. The importance of high ultimate elongation is also recognized in the standard testing procedure ASTM D3577, which requires an ultimate elongation of at least 650% percent for synthetic gloves, and at least 750% for natural rubber gloves. In the case of catheter balloons, a high ultimate elongation lowers the stress that is placed on the balloon when inflated and thereby helps prevent premature failure. It is well known that for any given rubber composition, sulfur-vulcanized articles exhibit higher elongation than do equivalent articles which contain only carbon-carbon crosslinks.
The following is a survey of disclosures that may constitute prior art relevant to certain aspects of the invention set forth herein. The relevance of each of these disclosures will be apparent from the succeeding sections of this specification and claims. All patents and published materials cited throughout this specification are incorporated herein by reference in their entirety.
The use of cis-1,4-polyisoprene latex compositions for use in medical devices or medical device components is well known. Preiss et al. in U.S. Pat. No. 3,215,649, disclose the use of a sulfur-vulcanized cis-1,4-polyisoprene. McGlothlin et al. in U.S. Pat. No. 6,329,444 disclose the use of sulfur-free, free-radical-cured cis-1,4-polyisoprene for use in dip-molded medical devices. Leeper et al. in U.S. Pat. No. 4,938,751 disclose the use of reinforced free radical crosslinked cis-1,4-polyisoprene in elastomeric bladders. The Leeper et al. patent addresses molded (non-latex) rubber articles, but still of fairly thin walls. Both the McGlothlin et al. and Leeper et al. patents cite the high level dimensional stability of the cured polyisoprene materials, primarily due to the carbon-to-carbon crosslinking. McGlothlin et al. state that tensile set values of less than 5% can be achieved, while Leeper et al. reveal that a low frequency hysteresis less than about 10% and a stress relaxation less than about 10% can be achieved. Neither McGlothlin et al. nor Leeper et al. disclose the use of sulfur in combination with organic peroxides to improve the physical properties of synthetic polyisoprene.
Zabielski et al. in U.S. Pat. No. 4,724,028 disclose the use of a free radical curing mechanism to cure medical injection sites made from cis-1,4-polyisoprene via an extrusion process. Noecker et al. in U.S. Pat. No. 6,051,320 disclose the use of reinforcing agents to improve the tensile strength of free radical cured natural rubber for use in medical devices. Noecker et al. admit that “. . . the sample rubber latex gloves according to the invention are somewhat inferior in tensile strength and modulus of elasticity than the conventional rubber latex gloves formed using sulfur and related curing agents.” The tensile strength cited by Noecker et al. for natural rubber is 21 to 24 MPa. There is no reference at all to synthetic polyisoprene. Neither the Zabielski et al. nor Noecker et al. patents provide any suggestion of combining sulfur and free-radical curing mechanisms to improve tensile strength.
Class in U.S. Pat. No. 6,245,861 states that compositions cured exclusively with peroxides are thought to have shorter crosslinks which are less flexible than comparable crosslinks from sulfur-cured compositions and therefore peroxide-cured compositions are believed to exhibit less resistance to abrasion and cut growth. While not directly referring to synthetic polyisoprene, Class addresses problems that generally arise with free-radical-cured rubber compounds.
The use of coagents has been suggested as a means to overcome the objections to pure peroxide cures. Typical coagents as disclosed by Class include trimethylolpropane trimethacrylate, triallyl isocyanate, pentaerythritol tetramethacrylate, trimethylolpropane trimethacrylate, triallyl isocyanate, pentaerythritol tetramethacrylate, and low molecular weight 1,2-polybutadiene. Class states that coagents can increase the modulus and hardness of a peroxide-cured composition. While suggesting the combination of both peroxide and sulfur in the same composition, Class does not suggest that sulfur is a coagent for the peroxide. Class specifically discloses however the use of traditional accelerators when sulfur is used. Class does not mention the use of polyisoprene, and does not mention medical device applications. In many medical device applications, the increase in hardness and modulus is not desirable, especially for thin-walled products such as condoms and gloves.
Blok et al. in U.S. Pat. No. 6,300,421 provide a comprehensive overview of the role of coagents in the curing of EPDM elastomers. Blok et al. further disclose the use of elemental sulfur as a coagent for peroxides in the curing of EPDM rubber. Also disclosed is the potential use of polyisoprene as a component of the EPDM formulation. Blok et al. further state that in order to minimize, or retard, the occurrence of side reactions, co-agent(s) may be used in combination with the peroxide curative to react with and stabilize the free radicals formed during the curing process. In this manner, a co-agent tends to improve the overall crosslinking efficiency and thereby lead to a higher cure rate and state of cure. This is well known to those having skill in such art. Blok et al. do not suggest that any carbon-sulfur bonds are actually formed. The sulfur is likely acting as a traditional coagent to help the efficiency, rate and state of cure, which will generally increase the tensile modulus and reduce the ultimate elongation of the rubber. Blok et al. do not disclose anything related to natural rubber or synthetic polyisoprene, or to medical devices or latex applications.
Magei et al. in U.S. Pat. No. 4,218,548 disclose the use of sulfur as a coagent for ethylene propylene rubber. As in Blok et al., there is no suggestion that the sulfur is acting as a vulcanizing agent. There is also no mention or suggestion by Blok et al. of the use of peroxide compounds with polyisoprene or for the curing of medical devices.
Sartomer Company, Inc., Exton, Pa., USA, manufactures a large number of products for use as peroxide coagents in curing elastomers. Sartomer has published a technical bulletin entitled “Coagent Selection for Peroxide Cured Elastomers.” While not specific to synthetic polyisoprene, the bulletin contains references to co-agents for elastomers in general. Table 15 of the bulletin provides a generalized cure property comparison between a peroxide-only curing system, an accelerated sulfur-only curing system, and seven different peroxide-coagent curing systems. The data show that tensile modulus and hardness both increase with the addition of coagents, as compared to both accelerated sulfur-curing systems and to peroxide-only curing systems. This is not desirable from the perspective of making highly elastic medical devices such as condoms, gloves, balloons, and surgical tubing. The bulletin does not disclose the possibility of using sulfur in combination with peroxide.
An article by McElwee and Lohr entitled “Comparing curing systems: peroxide-coagent versus sulfur-accelerator in polyisoprene” appears in Rubber World, Kippincott & Peto, Inc, Akron, Ohio, USA, Volume 225, No. 2, November 2001, pages 41-44. The article states that a peroxide-coagent curing system has the best characteristics of both peroxide and sulfur cure systems, i.e., high tensile strength, high tear strength, high modulus, and outstanding flex and heat-aged properties. While several acrylic and other coagents are disclosed, the use of sulfur as a coagent is not disclosed. Comparisons are made between sulfur-cure, peroxide-cure, and peroxide/coagent cure in terms of several physical properties. The comparisons show that the tensile modulus obtained with the peroxide-coagent system is higher than that obtained with the other cure systems, indicating that the peroxide-coagent system achieves a greater state of cure. The Shore A hardness is also shown to be significantly higher with the peroxide-coagent system than for either the accelerated sulfur-cure or the peroxide-only cure, results that are consistent with other prior art observations. The article does not reveal the advantage of using sulfur in conjunction with peroxide to cure polyisoprene in terms of achieving a lower modulus and higher elongation without increasing the hardness of the material.
Stevenson in U.S. Pat. No. 4,695,609 A1 discloses a process for preparing a vulcanized rubber article using sulfur vulcanization with a combination of accelerators which include a dihydrocarbyl xanthogen polysulphide and less than 0.4 part by weight of nitrosatable materials. The accelerator combination is disclosed for use with synthetic polyisoprene. The process described in the patent reduces the amount of nitrosamine formation during curing, and achieves a significant reduction in the use of toxic conventional nitrogen-containing accelerators, but does not allow for the complete elimination of such compounds. The use of peroxide and sulfur in combination for vulcanization of polyisoprene is not disclosed.
Stevenson et al. in U.S. Pat. No. 5,254,635 disclose a means for reducing the amount of nitrosatable compounds needed in rubber formulations. While not specifically citing synthetic polyisoprene, Stevenson et al. state that the use of potentially nitrosatable materials such as secondary and tertiary amines may need to be added as supplemental accelerators to provide for a satisfactory degree of cure when the rubber to be cured is a synthetic rubber. Stevenson apparently was still able to limit the amount of these undesirable substances to about 0.2 phr. While this is a low level, its is still an undesirable amount for the fabrication of medical devices and components. Again, there is no disclosure of the use of peroxide and sulfur in combination for vulcanization of synthetic polyisoprene.
Virdi in U.S. Pat. No. 6,162,875 discloses the use of zinc diisononyldithiocarbamate as a sulfur accelerator which is thought to produce safer nitrosamines that are likely to be non-mutagenic. Vulcanizates produced by the Virdi process still contain nitrosamines, however.
Puydak et al. in U.S. Pat. No. 5,073,597 disclose the use of sulfur as a coagent for peroxide in curing EPM and EPDM rubbers for use in making dynamically vulcanized alloys that can be processed by thermoplastic forming methods. While the inclusion of synthetic polyisoprene in the composition is disclosed, the role of the polyisoprene is not defined and no mention is made of vulcanization of the optional polyisoprene. No special characteristics are assigned to the peroxide-cured compositions that use sulfur as a coagent. Furthermore, the use of dynamic vulcanized rubber materials is limited and cannot be used to produce high tensile strength, low tensile set rubber materials.
Numerous methods for the vulcanization of peroxide-containing formulations are known. Most of these methods involve excluding oxygen from the rubber composition during the curing process. McGlothlin et al. in U.S. Pat. No. 6,329,444 disclose methods to protect thin films of organic peroxide-containing polyisoprene from oxygen exposure during vulcanization. Verlaan et al. in U.S. Pat. No. 4,808,442 teach several methods to protect organic peroxide-containing rubber compositions from degradation caused by oxygen attack of the rubber. Compression, transfer and injection molding are known methods of protecting such rubber compositions during the curing process.
Organic peroxide-cured rubber particles can be prevulcanized prior to being formed into shaped articles if oxygen is mostly excluded during the prevulcanization process. One such process, in which dicumyl peroxide is used to prevulcanize synthetic latex rubber particles is disclosed by Bayer AG (Obrecht) in WO 02/08328 A1.
Dillenschneider in U.S. Pat. No. 3,937,862 discloses tire sidewalls formed from a mixed sulfur and peroxide vulcanization system (Example 23) with an EPDM polymer having a relatively low molecular weight (Mooney viscosity of 84 at 100° C.). Dillenschneider concludes that the mixed vulcanization system offers no particular advantage over an all-peroxide vulcanization system. While Dillenschneider discloses the use of mixtures of rubbers, some of which may include polyisoprene and/or natural rubber, the patent does not disclose the use of a mixed sulfur and peroxide vulcanization system for polyisoprene-containing blends. All of the disclosed compositions include the use of nitrosatable rubber accelerators. Dillenschneider further states that the use of sulfur in very small amounts, such as from about 0.1 to about 0.3 phr, would be insufficient for vulcanization in the absence of both peroxide and an accelerator.
Wei et al. in U.S. Pat. No. 3,179,718 teach the use of a mixture of peroxide and elemental sulfur to vulcanize blends of highly saturated rubber with butadiene-acrylonitrile rubber. In comparative examples, Wei et al. refer to the curing of natural rubber with a combination of sulfur and peroxide. The form of natural rubber used by Wei et al. was smoked sheet rubber. When compounding natural rubber with a combination of 2 phr of sulfur and 4 phr of dicumyl peroxide, Wei et al. produced a vulcanizate with a tensile strength of 2360 psi and an ultimate elongation of 570%. While Wei et al. suggest that synthetic polyisoprene can be blended with other rubber material and then vulcanized with a sulfur/peroxide curing system, there is no mention of the use of synthetic polyisoprene alone. Nor do Wei et al. disclose the use of latex formulations, or state that the disclosed curing system produces a product with high tensile strength. Nor do Wei et al. mention avoidance of Type I or Type IV latex allergies. Still further, the only rubber articles that Wei et al. address are tire treads, windshield channels, and cable coverings. Thin-film rubber articles are not addressed or suggested.
Mitchell in U.S. Pat. No. 4,973,627 teaches the use of a tire sidewall composition that includes a mixed sulfur and peroxide vulcanization system. The optional inclusion of polyisoprene and/or natural rubber in the sidewall composition is mentioned. The patent expressly states that it is necessary to include sulfur accelerators in the manufacture of the disclosed product.
Podell, Jr., et al. in U.S. Pat. No. 3,813,695 disclose the application of acrylic hydrogel coatings to gloves to serve as donning aids. No disclosure is made of such coatings being used to eliminate the passage of oxygen during the peroxide curing of gloves.
The prior art indicates that thin-walled latex dip-molded rubber articles with a combination of excellent tensile strength, low 500% modulus, and high ultimate elongation, can only be obtained by vulcanization with sulfur in combination with a nitrosatable (i.e., secondary amine-containing) sulfur accelerator. The present invention overcomes this limitation.